The viscosity of lubricating oils varies with temperature. In general, oils are identified by a viscosity index which is a function of the oil viscosity at a given lower temperature and a given higher temperature. The given lower temperature and the given higher temperature have varied over the years but are fixed at any given time in an ASTM test procedure (ASTM D2270). Currently, the lower temperature specified in the test is 40.degree. C. and the higher temperature is 100.degree. C. For two engine lubricants with the same kinematic viscosity at 100.degree. C., the one having the lower kinematic viscosity at 40.degree. C. will have the higher viscosity index. The oil with the higher viscosity index undergoes less kinematic viscosity change between the temperatures of 40.degree. C. and 100.degree. C. In general, viscosity index improvers that are added to engine oils increase the viscosity index as well as the kinematic viscosities.
The SAE Standard J300 viscosity classification system does not specify the use of viscosity index to classify multigrade oils. At one time, however, the Standard did require that certain grades meet low-temperature viscosities that were extrapolated from kinematic viscosity measurements taken at higher temperatures, for it was recognized that oils that were exceedingly viscous at low-temperatures caused engine starting difficulties in cold weather. For this reason, multigrade oils which possessed high viscosity index values were favored. These oils gave the lowest low-temperature extrapolated viscosities. Since then, ASTM has developed the cold cranking simulator (CCS), ASTM D5293, (formerly ASTM D2602) a moderately high-shear-rate viscometer which correlates with engine cranking speed and starting at low temperatures. Today cranking viscosity limits, determined by the CCS, are defined in the SAE J300 Standard and viscosity index is not used. For this reason, polymers that improve the viscosity characteristics of lubricating oils are sometimes referred to as viscosity modifiers instead of viscosity index improvers.
Today, it is also recognized that cranking viscosity is not sufficient to fully estimate a lubricant's low-temperature performance in engines. SAE J300 also requires that pumping viscosity be determined in a low-shear-rate viscometer called the mini-rotary viscometer IV). This instrument can be used to measure viscosity and gel formation, the latter by the measurement of yield stress. In this test, an oil is slowly cooled over a two-day period to a specified temperature before viscosity and yield stress are determined. A yield stress observation constitutes an automatic failure in this test, while pumping viscosity must be below a specified limit to ensure that the oil will not cause an engine to experience a pumping failure during cold weather conditions. The test is sometimes referred to as the TP1-MRV test, ASTM D4684.
Numerous materials are used in the formulation of fully-formulated multigraded engine oils. Besides the basestocks, which may include paraffinic, napthenic, and even synthetically-derived fluids, the polymeric VI improver, and the pour point depressants, there are numerous lubricant additives added which act as antiwear agents, antirust agents, detergents, dispersants, and pour point depressant. These lubricant additives are usually combined in diluent oil and are generally referred to as a dispersant-inhibitor package, or as a "DI" package.
Common practice in the formulation of a multigrade oil is to blend to a target kinematic viscosity and cranking viscosity, which is determined by the specified SAE grade requirements in SAE J300. The DI package and pour point depressant are combined with a VI improver oil concentrate and with one basestock, or two or more basestocks having different viscosity characteristics. For example, for an SAE 10W-30 multigrade, the concentration of the DI package and the pour point depressant might be held constant, but the amounts of HVI 100 neutral and HVI 250 neutral or HVI 300 neutral basestock might be adjusted along with the VI improver until the target viscosities are arrived at.
The pour point depressant's selection normally depends on the type of wax precursors in the lubricant basestocks. However, if the viscosity index improver itself is prone to interact with wax precursors, it may be necessary to add an additional pour point depressant of a different type, or an additional quantity of the pour point depressant used for the basestocks to compensate for that interaction. Otherwise, low-temperature rheology will deteriorate, and TP1-MRV failures will result. The use of additional pour point depressant generally increases the cost of formulating an engine lubricant.
Once a formulation has been arrived at that has the targeted kinematic viscosities and cranking viscosities, the TP1-MRV viscosity is determined. A relatively low pumping viscosity and the absence of yield stress is desirable. The use of a VI improver which contributes little to low-temperature pumping viscosity or yield stress is very desirable in the formulation of multigrade oils. It minimizes the risk of formulating an oil that may cause an engine pumping failure and it provides the oil manufacturer with additional flexibility in the use of other components which contribute to pumping viscosity.
Viscosity index improvers that are hydrogenated star polymers containing hydrogenated polymeric arms of copolymers of conjugated dienes, including polybutadiene made by the high 1,4-addition of butadiene, were previously described in U.S. Pat. No. 4,116,917. U.S. Pat. No. 5,460,739 describes star polymers with (EP-EB-EP') arms as VI improvers. Such polymers produce good thickening characteristics but are difficult to finish. U.S. Pat. No. 5,458,791 describes star polymers with (EP-S-EP') arms as VI improvers. Such polymers have excellent finishability characteristics and produce oils with good low temperature performance but the thickening characteristics are diminished. It would be advantageous to be able to produce a polymer with good thickening characteristics and excellent finishing characteristics. The present invention provides such a polymer.